Subject of the present invention belongs to the field of protective electronic elements, used in safeguarding of other electronic elements and devices against voltage and current strokes, and against high and low-frequency disturbances.
Miniaturisation and increase of complexity of contemporary electronic elements and systems, as well as their increasingly wider application simultaneously increase their sensitivity to electronic discharge (ESD), voltage excess and strokes and frequency disturbances. These phenomena may disable normal functioning of separate components, decrease their stability and reliability and may even destroy them. In this way above phenomena jeopardise normal functioning of very complex and expensive electronic systems Sources of these phenomena may be various. For instance the largest source of electrostatic discharge is man itself in the process of manipulation with electronic devices. Most typical source of voltage strokes is lightning, which induces charge in electrical and telecommunication lines, this charge being transferred through the lines to electronic elements. And some of electronic elements themselves (e.g. electromotor, relay) induce disturbances of various frequencies and emit them in their closest surrounding. Thus each separate electronic component and its part shall be protected with special components that provide such protection. On the other hand this means that disturbance emission shall be prevented at the source.
Only use of components, with following properties may provide protection against ESD and voltage strokes: non-linear I-U characteristic, short response time and ability to absorb larger amounts of energy. Such characteristics are available in some elements, like Zener diode, multilayer ZnO polycrystalline diode (Varicon), varistor, and condensers with varistor characteristic or self-limited breakthrough.
The simplest elements for protection against frequency disturbances are condensers with capacitance up to do 100 nF for high frequencies and in case of lower or radio frequencies with capacitance up to 2000 nF.
Consequently it is clear that in order to protect against all mentioned undesired phenomena, such protective component shall have all required properties: non-linear I-U characteristic, short response time, ability to absorb larger amount of energy, and adjustable capacitance in range from 10 to 2000 nF. Beside that it must be smaller than any existing solutions and shall offer surface mounting option with terminal leads.
Subject of this invention, multifunctional component has exactly such properties.
Beside the most expensive solution, namely use of two discrete elements in parallel electrical connection e.g. combinations Zener diodexe2x80x94condenser or varistor-condenser, there are some other contemporary solutions one of which is condenser with varistor properties as described in European Patent 418394A. Mentioned condenser is based on SrTiO3, which provides high value of dielectric constant (xcex5 greater than 20000). Multilayer manufacturing technology provides wide scope of capacitance (10-2000 nF) at relatively small dimensions. However its worst side is, bad varistor characteristic and low value of non-linearity coefficient, soft knee and also high leakage current, high value of breakthrough voltage temperature coefficient and very narrow range of operating voltage. Further more materials for manufacturing such elements and production technology are very expensive. In U.S. Pat. No. 5,146,200 hybrid bond between multilayer chip varistor and multilayer chip condenser is disclosed. Physical bond of these two elements is achieved with gluing and parallel electrical connection is achieved with soldering.
Components as self-limiting multilayer condenser as shown in U.S. Pat. No. 4,729,058 and multilayer ZnO polycrystalline diode (Varicon diode) have very non-linear I-U characteristic and high self capacitance, which may within real dimensions reach up to 100 nF. In that way these two mentioned components provide protection against ESD and voltage strokes and effectively filter high frequency disturbances, but not low frequency disturbances.